Polyurethane systems have been widely used in underground coal mining in order to consolidate and seal geological and loosened rock and soil formations; see "Gluckauf" (1968), pages 666-670; "Gluckauf" (1977), pages 707 to 711; "Bergbau" (1977), pages 124-129; DE-A-17 58 185 and DE-A-17 84 458.
Polyurethane systems are not, however, suitable for rock formations carrying water, since water reacts with polyisocyanate with the result that the stoichiometric ratio of the reactants is crucially unbalanced. Moreover, the product mostly formed from water and polyisocyanate is polyurea which does not bind cracks and fissures.
Another disadvantage of using polyurethane in coal mining is the fact that the cured product is highly inflammable.
A process for consolidating and sealing geological and loosened rock, soil and brick formations and coal, which makes use of the reaction of polyisocyanates with water glass is known from DE-A-29 08 746. In this known process, water glass solutions and polyisocyanates are intimately mixed and this emulsion is allowed to cure in the formation to be consolidated.
In the known process, up to 2 weight percent of accelerators known in polyurethane chemistry, are added to the mixture. Preferred accelerators are organometallic compounds such as dibutyl tin dilaurate or tert. amines, such as triethylamine. Moreover, known blowing agents, such as acetone, methylene chloride, monofluorotrichloromethane, dichlorodifluoromethane and butane are used in amounts of up to 30 weight percent, based on the polyisocyanate/water glass solution mixture. Finally compounds having at least one reactive group with respect to the polyisocyanate, preferably polyols, are added to the mixture in an amount of up to 30 weight percent, based on the water glass solution.
However, because of the insufficient physical properties and mechanical strength of the resulting organomineral products, the process known from DE-A-29 08 746 does not render the coal or rock and brick formation satisfactorily consolidated. For instance, known non-expanded reaction products of polyisocyanate and water glass show insufficient tensile bending strength after 2 hours at 50.degree. C. After 8 days they still show tensile strength bending values which are lower than the corresponding values of purely organic products (polyurethane). The latter reach their curing maximum after about 4 hours and do not get any harder after this time. While in polyurethane systems shorter curing times are possible, they are critical in view of the exothermic reaction, as they can lead to the self-ignition of the coal bed.
When taking a closer look at reaction systems consisting of water glass solutions and NCO-group bearing preparations, the difficulties in the formulation will become apparent. On the one hand, the complicated chemical reaction scheme has to be made controllable operation-wise. On the other hand, the end product must meet very specific requirements. Since the requirements are frequently diametrically opposed, the common denominator found is insufficient. So far, these facts have considerably restricted the use of reasonably priced water glass solutions as organic/inorganic systems.
In reaction systems containing a polyisocyanate and an aqueous water glass solution a stoichiometric NCO/OH ratio cannot be achieved so that the reaction proceeds in an uncontrollable manner. Therefore, the polyisocyanate cannot be expected to form in any way an organic polymer structure of any practical use. For this reason, the reaction of polyisocyanate in water glass solutions is of technical interest only in as far as gaseous CO.sub.2 which can be considered as hardener and coagulant for the water glass is released when R-NCO and water react with each other. The low molecular urea product resulting from the polyisocyanate remains distributed in the mineral structure of the water glass as hard filler in the form of very fine particles.
In practical formulation, another problem arises from the excess amount of gaseous CO.sub.2. DE-A-17 70 384, for instance, see page 6, lines 9 to 14, already points out the necessity to observe the stoichiometric ratio of the reactants as much as possible. However, it does not indicate how this is to be achieved.
Moreover, in connection with the stoichiometric ratio, only the R--NCO/OH ratio is considered. No thought is given to the importance of a reaction ratio of Me.sub.2 /SiO.sub.2 /CO.sub.2 --defined in whatever way. The next aspect pointed out is that with a higher polyisocyanate portion, the reaction proceeds rapidly and also tends to froth up. In view of these facts, the utility of the products obtainable according to said publication is considerably restricted.
From the mold manufacture where molds from sand and sodium water glass are prepared, it is known that an excess amount of gaseous CO.sub.2 used to harden water glass will adversely influence the stability of the molding composition. This is an effect which occurs in a reaction mixture with water glass on account of a high excess amount of polyisocyanate.
In DE-A-24 60 834, a catalyst which is capable of trimerizing polyisocyanate in a manner known per se is added to the reaction mixture water glass/polyisocyanate. However, the process described in said application merely serves to prepare organomineral foams. DE-A-24 60 834 does not give clear teaching as to the amcunt of catalyst to be used; in the examples using 2,4,6-tris-(dimethylaminomethyl)-phenol as catalyst and polyphenolpolymethylenepolyisocyanate with an NCO-group content of about 28%, said catalyst is used in an amount of about 18 to 36 mmole per mole of NCO groups. In the case of other catalysts or polyisocyanates the ratio is even substantially higher.
It is the primary object of the invention to provide a process for consolidating and sealing coal and/or natural or synthetic rock, soil and brick formations, for instance in mining or tunnelling, by reacting polyisocyanates and aqueous alkali silicate solutions in the coal or rock and soil formation to be consolidated, which process leads under the existing conditions to a satisfactory consolidation of the corresponding formation.
It is a further object of the invention to provide a process of the above-mentioned type by which the consolidated formation is imparted excellent strength, especially tensile bending strength.
It is a further object of the invention to provide a process of the above-mentioned type which leads to a satisfactory consolidation of the formation showing excellent strength within very short time.
It is still another object of the invention to provide a consolidation and sealing process by which mining and tunnelling work can be continued within short time without danger for the workers.
These and other objects and advantages of the invention will become apparent to those skilled in the art from the following description of the invention.